A high-temperature multinuclear NMR study of Na3AlF6-FeO and Na3AlF6-Fe2O3 melts

摘要

In the Hall-Heroult process for the industrial production of aluminum, iron oxide impurities are known to lower the current efficiency as well as the metal quality. Iron can be present in the di- or trivalent form. The nature of dissolved FeIII and Fell species and the reactions taking place in cryolitic melts have been investigated by high-temperature NMR spectroscopy. The evolution of Al-27, Na-23, and F-19 NMR chemical shifts are reported in the Na(3)AIF(6)-FeO and Na(3)AIF(6)-Fe2O3 systems for different iron oxide contents. They express the formation of oxofluoroaluminate species. For the Na3AIF6-FeO system, the F-19 signal appears only after 14 minutes at 1020 degrees C. The line position is shifted with heating time. This evolution is associated with a narrowing of the Al-27 signal due to a decrease in the content of paramagnetic compounds in the sample. This effect is probably caused by the evaporation of an Fe-II compound, as already mentioned in the literature. For the Na(3)AIF(6)-Fe2O3 system we have never observed the F-19 NMR spectra over the full heating time (ca. 30 min). This could be due to the influence of paramagnetic Fe-III nuclei, probably in FeF63- species present in the melt.